1. Field of the Invention
The present invention relates to a heat exchanger, and more particularly, to a heat exchanger for use, for example, as an evaporator for an automobile cooling apparatus.
2. Description of the Prior Art
In general, the evaporators for such use are divided into a variety of types in accordance with how many times a cold fluid passes through the core portion of the evaporator; for example, a two-pass system, a three-pass system and so on.
In order to explain the background of the invention in detail, reference will be made to FIG. 9:
The illustrated evaporator is a two-pass system, in which a plurality of fluid paths 30 are arranged in parallel with air paths being interposed between each path 30 and the next. The cross-sectional areas of the fluid paths 30 are equal.
Half of the fluid paths 30 are connected to an inlet header 31 at their one ends, and the remaining half thereof are connected to an outlet header 32 at their other ends. The opposite ends of all the fluid paths 30 are connected to a common header 33. In this evaporator the cold fluid passes through the core portion of the evaporator with a single U-form turn, which is illustrated as t.sub.1 in FIG. 9(B). This means that the fluid passes twice in the core portion along the U-form pathway 40.
The evaporator shown in FIG. 10(A) is a multiple pass system (8 passes). Both ends of the paths 30 are connected to the common header 33 so that each path extends in a U-form t.sub.2 as shown in FIG. 10(B). The cold fluid is introduced into the evaporator through the inlet header 31, and passes through the U-form path t.sub.2. The cold fluid goes out of the outlet header 32.
The evaporator shown in FIG. 11(A) is a one-pass system. The fluid is introduced into the evaporator through the inlet header 31, from which the fluid is distributed into many paths 30, and gathers in the outlet header 32. In this way the fluid passes only once in the core portion. The reference numerals 34 and 35 denote an inlet port and an outlet port, respectively.
In the evaporator of FIG. 11(A) the fluid tends to flow faster as it comes nearer the inlet and outlet ports 34 and 35. As a result, the efficiency of heat exchange reduces in the areas far from these areas. In this example the fluid flows diagonally along the path t.sub.3 as shown in FIG. 11(B), the path t.sub.3 connecting between the inlet port 34 and the outlet port 35. In the areas 36 and 37 the efficiency of heat exchange reduces in comparison with the areas 34 and 35 because of the relatively small amount of the fluid.
Another disadvantage is that the fluid is flown out of the evaporator before it is fully used as a coolant because of the shortest path connecting between the inlet port 34 and the outlet port 35. Consequently, the efficiency of heat exchange reduces. On the other hand, in a multiple pass system, such as a two-pass system or a three-pass system, heat exchange takes place evenly throughout the paths, thereby increasing the efficiency of heat exchange.
However, a disadvantage of the multiple pass system is that the fluid is likely to become choked near the outlet because of the fact that the fluid is introduced in an atomized form into the pathway 40 through the inlet header 31, and that the atomized fluid is gradually gasified by absorption of external heat, that portion of gasified fluid varying from 20% at the inlet header, 50% in the middle section, and 100% at the outlet header 32. Owing to gasification the fluid expands in the paths, thereby causing friction against the inside walls of the paths. This prevents the fluid from flowing smoothly, particularly when it comes near the outlet. To overcome the frictional resistance, and a high load occurring because of pressure drop at the inlet side of the evaporator, the compressor must be stepped up so as to supply a sufficiently pressurized fluid to the condenser. Furthermore, the heat exchange efficiency is likely to reduce because of the `dry-out` in the paths.